Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

The performance of the Li-mediated ammonia synthesis has progressed dramatically since its recent reintroduction. However, fundamental understanding of this reaction is slower paced, due to the many uncontrolled variables influencing it. To address this, we developed a true nonaqueous LiFePO4 reference electrode, providing both a redox anchor from which to measure potentials against and estimates of sources of energy efficiency loss. We demonstrate its stable electrochemical potential in operation using different N2- and H2-saturated electrolytes. Using this reference, we uncover the relation between partial current density and potentials. While the counter electrode potential increases linearly with current, the working electrode remains stable at lithium plating, suggesting it to be the only electrochemical step involved in this process. We also use the LiFePO4/Li+ equilibrium as a tool to probe Li-ion activity changes in situ. We hope to drive the field toward more defined systems to allow a holistic understanding of this reaction.

is the corrected electrode potential, is the raw potential measured during experiments and is the current passed during experiments.
is the uncompensated resistance between the working (or counter) and reference electrode, collected from the PEIS measurements.
is obtained here through fitting of the experimental data to a suitable equivalent circuit. (c) Equivalent circuit used to fit the impedance spectra and extract as well as other impedance contributions such as: the charge transfer resistance, the double-layer capacitance, and the warbug impedance describing diffusion processes. 11

Coin cell assembly
Within an Ar-filled glovebox, a Li disc was mounted on stainless steel spacer and spring, placed in the negative case of a coin cell. A 18 mm diameter disc was cut out of the active materials sheets (LiFePO4 or Li4Ti5O12), then placed in the opposite positive case and covered with a separator, which was wetted with 70-100 µL electrolyte (1 M LiNTf2 in THF). ( Figure   S 1a) Cell was closed with a 7-bar press.

Setup for electrochemical experiments
The 3-electrode sandwich cell used in this work consists of: (i) a 1 cm 2 Mo foil working electrode, polished, dipped in 4 M HCl and sonicated in ethanol for 10 min, (ii) a Pt mesh counter electrode on a 1 cm 2 Pt foil, (iii) a reference electrode -either a Pt wire or the previously prepared LiFePO4 or Li4Ti5O12 disc, shaped into a ring by punching a 8 mm diameter hole at its centre -. Within an Ar-filled glovebox, the cell was assembled, with the reference midway through the two other electrodes (1.8 cm between each). Gas-tight compartments were filled with 4 mL electrolyte, and leak tested by passing Argon through.
The cell was then saturated with THF-pre-saturated N2, bubbling for 30 min at a rate of 4 mL.min -1 before any experiment, then turned down to 1 mL.min -1 . All the electrochemistry was performed in the Argon glovebox. After experiments, cell was cleaned with EtOH out of the glovebox and boiled in ultrapure water (>18.2 MΩ, Sartorius), then dried in an oven at 70°C.
1.4. Measuring the drift in potential of a studied reference electrode To assess the extent of an undesired drift in reference electrode potential, we use the ferrocene-ferrocenium redox couple, an internal reference redox system with a defined 1 electron redox equilibrium (Figure 2a), approved as a reference for non-aqueous systems. 36 In every test condition, a voltammogram of a 1 M LiNTf2 and 10 mM ferrocene in THF electrolyte was recorded, at a 50 mV.s -1 rate. / + , the average between the potentials at which peak cathodic and anodic currents are reached (Figure 2a), is an estimate of its half-peak potential. 36 An important sidenote: during electrolysis, an unavoidable passivation layer forms at the working electrode ( Figure 1). To avoid resistive contribution of this layer to / + , postelectrolysis measurements ( Figure 2d) were done after replacing the working electrode with a fresh one.

Chronoamperometric electrolysis procedure
After cell assembly, the cell was purged with N2 for 30 min at a rate of 4 mL.min -1 before any experiment, then turned down to 1 mL. Sometimes, the sodium salicylate powder was contaminated with ammonia salts impurities.
To purify it from these interferents, 40 g sodium salicylate was dissolved in 300 mL DI water, to which 50 mL of 6 M aqueous HCl was added dropwise under constant stirring. The salicylic acid precipitate was filtered and washed with three times 200 mL ultrapure water, then dried under vacuum at 40°C. For 20 g of obtained salicylic acid, the solid was dissolved in 35 mL sodium hydroxide 4 M, to which were added 580 mL of sodium nitroprusside 50 mM solution, then completed with ultrapure water to 58 mL.

Ammonia quantification method
Electrolyte post electrolysis was collected, with its total volume measured. 3 x 400 L were collected alongside 400 L of previously saved pristine electrolyte blank in separate vials. 20 L 4 M aqueous HCl was added to each one of the four vials to trap NH3 as NH4Cl, which were then placed in a 70°C water bath for 1 h to evaporate solvents. Concentrates were then redissolved in 2 mL ultrapure water.
Resulting samples were dispatched in 1 mL portions in UV-vis cuvettes, to quantify ammonia via the salicylate method: a colorimetric detection method based on the complexation of ammonia with sodium salicylate to create a blue dye. For this method, a 1 mL sample was diluted to a volume of 2 mL with ultrapure water. Then, 280 L of the "salicylate -catalyst" solution was added, followed with 280 L of the sodium hypochlorite alkaline solution.
Samples were left to age in the dark for 45 min, then characterised by UV-vis absorption spectroscopy, measuring absorbance of light between 500 nm and 900 nm wavelengths, and measuring the difference in absorbance between the maximum (655 nm) and baseline (900 nm).
In this work, we couple this quantification experiment to the quantification method of       (Figure 2b). (d) Working electrode potential recorded between ferrocene cycling steps, showing partial to full relaxation to OCP between each experiment.